Generally, a polyimide (PI) film is obtained by shaping a polyimide resin into a film. The polyimide resin is a highly heat-resistant resin which is prepared by performing solution polymerization of aromatic dianhydride and aromatic diamine or aromatic diisocyanate to prepare a polyamic acid derivative and then performing imidization by ring-closing dehydration at a high temperature. Having excellent mechanical, heat resistance, and electrical insulation properties, the polyimide film is used in a wide range of electronic materials such as semiconductor insulating films, electrode-protective films of TFT-LCDs, and substrates for flexible printed wiring circuits.
Polyimide resins, however, are usually colored brown and yellow due to a high aromatic ring density, so that transmittance in a visible ray region is low and the resins exhibit a yellowish color. Accordingly, light transmittance is reduced and birefringence is high, which makes it difficult to use the polyimide resins as optical members.
In order to solve the above-described limitation, attempts have been made to perform polymerization using purification of monomers and solvents, but the improvement in transmittance was not significant. With respect thereto, in U.S. Pat. No. 5,053,480, the transparency and hue are improved when the resin is in a solution or in a film form using a method using an aliphatic cyclic dianhydride component instead of aromatic dianhydride. However, this was only an improvement of the purification method, and there remains a limitation in the ultimate increase in transmittance. Accordingly, high transmittance could not be achieved, but the thermal and mechanical deterioration resulted.
Further, U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083, 5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036, and 6,232,428 and Korean Patent Application Publication No. 2003-0009437 disclose a novel structure of polyimide having improved transmittance and color transparency while thermal properties are not significantly reduced using aromatic dianhydride and aromatic diamine monomers having a substituent group such as —CF3 or having a bent structure in which connection to a m-position instead of a p-position occurs due to a connection group such as —O—, —SO2—, or CH2—. However, this has been found to be insufficient for use as materials for display devices such as OLEDs, TFT-LCDs, and flexible displays due to limitations in terms of mechanical properties, heat resistance, and birefringence.